Pearl is one of the most valuable gems. Large, perfectly shaped pearls rank in value with the most precious stones. But pearls are different from other gems. Most gems are minerals that are mined from beneath the earth. But pearls are formed inside the shells of oysters. Mineral gems are hard and usually reflect light. However, pearls are rather soft, and they absorb, as well as reflect, light.
How pearls are formed
Oysters and other shell-forming mollusks make a special substance, called nacre << NAY kur >>, that lines the insides of their shells. This smooth lining is called the nacreous layer, or pearly layer, and it is often lustrous. It is formed by cells from a fleshy body organ called the mantle. When a foreign substance, such as a bit of shell or a tiny parasite, enters the body of the mollusk, the mantle cells begin to work. They cover the invading substance with thin sheets of nacre. They build successive circular layers of nacre until the foreign body is enclosed in the shell-like substance, forming the pearl.
The pearl has the same luster and color as the lining of the shell of the mollusk. But few pearl-forming mollusks produce the beautifully colored nacre that is essential for valuable pearls. Valuable pearls come from some species of oysters and other mollusks that live in tropical seas. Some species of mussels found in rivers also produce precious pearls. Edible clams and oysters have dull shells, so their pearls are without luster. As a result, they have no value as gems.
Characteristics of pearls
When a pearl is cut in two and examined under a microscope, the layers can be seen. Because the layers are concentric (formed in a complete circle around the central substance), the cut pearl looks like a sliced onion. The layers are made up of little crystals of a mineral substance called aragonite, a form of calcium carbonate. They are held in position by a cartilagelike material known as conchiolin << kong KY oh lin >>. The tiny mineral crystals overlap and break up any light that falls on them into little rainbows of color. This gives pearls their iridescence, which jewelers call orient.
Conches, clams, and most edible oysters usually do not make pretty pearls because their aragonite crystals are too large. Even though the pearls may be of beautiful pink, white, or purple color, they lack iridescence.
Color.
Oriental pearls, so called not because they come from the Far East but because they are iridescent, may also have color. These pearls may be “black,” pink, orange, gold, cream, or white. “Black” pearls are really a dark gray. They are among the most valuable of all pearls.
Shape
of a pearl is as important as its color. Round pearls, suitable for necklaces, are the most valuable. Next in value are the button-shaped and drop-shaped pearls. These are often used for earrings. Matched pairs of these pearls are even more valuable than pairs of unmatched single ones. Pearls with irregular shapes are called baroques. They are less valuable than the others. Pearls made by a kind of snail called the abalone have wonderful color and luster but are almost never symmetrical.
Blemishes.
Perfect pearls and pearls with only one blemish are the most valuable. Sometimes pearl blemishes can be removed if the flaw is not too deep. Specially trained workers called peelers carefully scrape away the blemished layers. When they have removed the flaw, the pearl is smaller, but perfect. Such a pearl is worth more than the original large, but blemished, pearl.
Matching pearls.
The matching of pearls to make a pair or a string makes the finished piece more expensive than the total cost of the individual pearls. Each added pearl must be like all the others in color and orient. Often it must be of the same size as the others. It must have no more than one tiny blemish. One blemish is acceptable because the pearl can be drilled for mounting at the blemish. Before people began to produce cultured pearls, it could take many years to fill a necklace of matched pearls.
Value.
The cost of pearls sold in large quantities is determined by their weight. In a piece of jewelry, the value of a pearl is determined by its size, color, and luster. For example, matched pearls in a necklace cost more than the total of their individual values.
Kinds of pearls
Natural pearls.
Until the 1940’s, the chief pearl-oyster beds were found in the Persian Gulf, near the island country of Bahrain. Other natural pearl-oyster beds were located in the South Pacific Ocean. Thousands of oysters had to be collected to produce even a small handful of pearls. For this reason, natural pearls were extraordinarily expensive. Today, few natural pearls are harvested for jewelry because the farming of cultured pearls produces pearls more cheaply.
Cultured pearls
are real pearls made by oysters. They usually can be distinguished from natural ones only by tests made in laboratories. The cultured pearl has a larger central body around which the layers of nacre form. The cultured pearl also has fewer and thicker layers of nacre. Thicker layers of nacre increase the value of the pearl.
Cultured pearls were first produced by inserting in an oyster a bead made of mother-of-pearl, the nacre secreted by certain inedible clams and oysters (see Mother-of-pearl). A small amount of mantle tissue from another oyster also was inserted. The process was developed by Kokichi Mikimoto of Japan in the early 1900’s. So successful was this process that the cultured-pearl business became much larger than the trade in natural pearls. Today, most cultured pearls are produced in Japan.
To produce cultured pearls, young oysters are planted in carefully selected oyster beds. When the oysters are 3 years old, they are taken from the beds to special production plants. There, trained people open the oysters’ shells and insert tiny pellets made of nacre or made from mussel shells. The workers then place the oysters in wire cages that will protect them from enemies. The cages are suspended from rafts and lowered into calm, protected waters near the shore.
Twice a year attendants raise the cages and remove seaweed and barnacles from the oysters. Progress of the oyster and the care given it are recorded on small metal tags attached to the cage. From one to three years after the pellets and tissue are inserted, the oyster is removed from the cage and its shell is opened. There is a valuable pearl in about 1 out of every 20 oysters opened. The pearl is washed, graded, and polished before it is sent to the market.
Imitation pearls
are manufactured. Usually, manufacturers coat glass beads with a substance known as pearl essence. This substance, sometimes known by its French name essence d’orient, is a creamy liquid extracted from fish scales. Herring scales usually furnish the main ingredient. Imitation pearls can be recognized by the little loose flaps of dried pearl essence surrounding the hole. Often, a little of the glass bead that the pearl essence has failed to cover can be seen at this place on the pearl.
Care of pearls
Because pearls are soft, they are easily scratched by such hard gems as diamonds. Pearls should always be put away carefully, out of contact with other jewelry. Pearls contain an organic material, conchiolin. This material dries out in time, or it can be destroyed by high temperatures. The aragonite crystals that make up the layers of nacre dissolve very quickly in acid. Perspiration sometimes contains acid. Therefore, jewelry made of pearls should be washed and dried gently after it is worn.
